1. Field of the Invention
The present invention relates to a method for driving a solid-state image pickup apparatus comprising e.g. a CCD register, a solid-state image pickup device and a camera comprising the solid-state image pickup device.
2. Description of the Related Art
The number of pixels in the solid-state image pickup device has remarkably increased with the progress of technology. With such an increase of the number of pixels, a function to reduce the amount of output data during one frame period as the need arises is strongly desired.
For example, in an electronic still camera, when taking a picture, priority is given to the resolution of a still picture for causing the CCD image pickup device to output e.g. 500 lines at a speed of e.g. 30 frames/sec. On the other hand, when viewing through an electronic finder, priority is given to the dynamic resolution for causing the device to output 250 lines at a speed of 60 frames/sec.
Unfortunately, according to this method, signals for the remaining 250 lines are not used and discarded.
Contrary to this, the applicant of this application has previously invented a technique for deriving signals from adding together signal charges apart by two pixels from each other in the vertical direction within a vertical transfer register (see Patent Gazette of Laying-Open No. 9-55952).
Due to this technique, it has become possible, in a solid-state image pickup apparatus having a color filters arranged repeatedly in a cycle of two vertical pixels, to mix signal charges of two pixels without discarding the signal charges and at the same storing time.
FIG. 1 is a conceptional diagram of the signal transfer by the CCD solid-state image pickup apparatus in this case.
Concerning the color filters, what is called Bayer's arrangement color filters are employed in which they are arranged in a two pixel cycle in both vertical and horizontal directions, greens G within each cycle being arranged diagonally in a checkered pattern, blues B and reds R being arranged in the remaining checkers, as shown in FIG. 2.
Letters in FIG. 2 denote colors of the filters (red R, green G, blue B) and numerals denote coordinates of pixels represented by row and column numbers (mn for the m-th row and the n-th column), respectively.
In FIG. 1, circle marks indicate positions of the weight center of the added signals and letters within the circle marks denote corresponding colors (red R, green G, blue B). Further, symbols shown outside the circle marks denote coordinate positions of the added components in FIG. 2.
First of all, signals corresponding to the first row G11, R12, G13, R14, etc. and signals corresponding to the third row G31, R32, G33, R34 etc. are added together inside the solid-state image pickup device to form signals having the weight centers in the second row of the color filters (G11+G31, R12+R32, G13+G33, R14 R34, etc.).
Also, signals corresponding to the second row B21, G22, B23, G24, etc. and signals corresponding to the fourth row B41, G42, B43, G44, etc. are added together inside the image pickup device to form signals having the weight centers in the third row of the color filters (B21+B41, G22+G42, B23+B43, G24+G44, etc.).
By using this method, signal charges of four pixels in the vertical direction can be made two signals and therefore the number of lines in the vertical direction can be made half, thus allowing the amount of data in one frame to be reduced.
According to the driving method described above, however, while the number of lines in the vertical direction can be made half and the amount of data in one frame can be reduced, a balance of resolutions in the horizontal direction and in the vertical direction will deteriorate in case of square grid pixels.
Specifically, though not shown in FIG. 1, because signals corresponding to the fifth row are added to those corresponding to the seventh row to form signals having the weight centers in the sixth row, these signals will be located four pixels apart from the same color signals which are derived from adding signals corresponding to the first row to signals corresponding to the third row to form signals having the weight centers in the second row.
Accordingly, for example, intervals between the same color pixels are two pixel intervals in the horizontal direction, whereas such intervals in the vertical direction are four pixel intervals, thus making the vertical resolution lower than the horizontal resolution.
Moreover, when the amount of data in one frame is to be reduced further using this method, the balance of the horizontal and vertical resolutions will become worse. For example, when one million, three hundred thousand pixel CCD having a normal speed of 15 frames/sec is operated at a speed of 60 frames/sec, the vertical resolution will go to one fourth.
Because it is necessary to reduce further the amount of data in the horizontal direction in order to overcome the foregoing problem, the present inventor proposed a method for driving the solid-state image pickup apparatus which enables the amount of data in the horizontal direction to be reduced by applying the aforesaid method for reducing the amount of data in the vertical direction.
FIG. 3 is a conceptional diagram of the signal transfer in the solid-state image pickup apparatus where the amount of data in the horizontal direction is reduced in addition to the reduction of the amount of data in the vertical direction. The arrangement of color filters is the same as that of FIG. 2.
In FIG. 3, circle marks denote positions of the weight center of the added signals and letters within the circle marks denote corresponding colors. Additionally, symbols shown outside the circle marks denote coordinate positions of the added components in FIG. 2.
Signals corresponding to four pixels G11, G13, G31, G33 of green G located in the left lower part of FIG. 2 are added together inside the solid-state image pickup device to form a single signal having the weight center in the position G22.
Likewise, signals corresponding to four pixels of blue B, namely, B21, B23, B41, B43 form a single signal having the weight center in the position R32. Signals corresponding to four pixels of red R, namely, R12, R14, R32, R34 form a single signal having the weight center in the position B23. Signals corresponding to four pixels of green G, namely, G22, G24, G42, G44 form a single signal having the weight center in the position G33.
According to this method, even when the color filters in a 2×2 cycle are used, it was possible to reduce the number of samples to one fourth inside the CCD solid-state image pickup device and further to solve the problem of asymmetry between the horizontal direction and the vertical direction.
By the way, when constructing the solid-state image pickup apparatus which is capable of switching between a normal outputting process and the outputting process for reducing the number of samples as the need arises, it is preferable to use the same algorithm for processing signals in those two outputting processes, if possible.
Furthermore, if samples are equal in number, it is preferable that a spacial distance relation among sample points is uniform, which enables the higher resolution and enable the conventional method for processing signals to be applied, thereby giving an advantage to make simpler of the signal processing.
According to the solid-state image pickup apparatus illustrated in FIG. 3, the weight center of samples (the position of G22) of the added signals of G11, G13, G31, G33 is one pixel apart from the weight center of samples (the position of B23) of the added signals of R12, R14, R32, R34, whereas the weight center of samples (the position of B23) of the added signals of R12, R14, R32, R34 is three pixels apart from the weight center of samples (the position G26) of the added signals of G15, G17, G35, G37.
In other words, it is possible to extract from signals output by the CCD solid-state image pickup apparatus signals in the same order as that of the original arrangement of color filters, whereas the symmetrical nature with respect to the parallel spacial distance relation carried by the original color filters is spoiled.
Thus, due to the non-uniformity of spacial distance relation in the arrangement of sampling points, it was difficult to apply the conventional algorithm in processing signals.